Volume compression device for sound reproducers



July 9, 1940 G. l.. DIMMICK 2,207,243

VOLUME COMPRESSION DEVICE FOR SOUND REPRODUCERS Filed April 14, 1958 L k /0 a 6195 P//aro @EM 0 k /z fg b 6 my l Smaclitor drrorneg Patented July 9, 1940 Y ,l y i UNITED STATES PATENT oFFlca voLpivm v ooMPnEssIoN n-Evlon ron .soUNn Rnritonuouns Glenn L. nimlmiek, naadnela, uN'.v J., assignor to Radio Corporation of America, a corporation of' Delaware Application April `14, 1938',- -Serial No. 202,085 6 Claims. (Gl.V Z50-415)' 'Ihis invention relates to volume compression with my invention plotted against volume corndevices and more particularly to a type of volume Apression in decibels, and, I

compression devices peculiarly adapted to use Figure 4 is a schematic diagram of my invenin photoelectric sound reproducers. tion as applied to a single photocell. y 5A In some sound reproducing devices used in Referring rst to Figure 1, apush-pull photo- 5 theatres, trouble has been heretofore encountered cell lli having cathodes II and I2 andl a com- .f

due to the reverberation characteristics of the monanode circuit I3 is supplied with current theatres and this trouble has been' particularly from an` appropriate source of supply iii, such noticeable where variable area sound records, as a B battery or an vequivalent source of supply.

which have a very wide range'of both amplitude The output from the DhOGOCell iS passed through 1I.)y

and frequency response, are used. 4The trouble the primaries I5 and I of a push-pull transi.; is not so noticeable in variable density recordformer, from the secondary Il of which current ing due to the limitations of the photographic is led to an appmprate amplfel- 111 the-'Permaterial which tends to flatten out the response OYnaIlCe 0f, my nVeIltiOD, insert a reSStOI I8 l in accordance with the shape of the H and D between the negative side of the source of cur- 16 u ,curve for the particular material used. rent Iii and the transformer primaries I5 and Such difficulties have sometimes been cpmit, the positive side of the said source of current pensated for in specific theatres by the addition being COHIIBGS- aS Usual t0 the anode O the of volume compressors in the thermionic ampliphOtOCell.

2Q ers used which compress the volume by making A gas-type photocell is preferably used. f l 20 the ampliiication an inverse function of the When the emission of the photocell cathodes volume. il and I2 is modulated by means of a class A The present invention provides a very much push-pull sound record, current flows through simpler but-nevertheless equally or more effecwindings ,I5 and I6. The current through `lthe tive means for securing such volume comprestwo windings I5 and `Iiincluding a direct-cur-- 25 sion and consists in the insertion of a very high rent component which passes through the reresistance in the lead from the negative side of sistor I-8 and audio frequency component which the polarizing supply for the photocell and bepasses through the two windings but not through tween the said supply and the output transtheresistor I8. The amount of direct current former. required by the photocell, however, varies in ac- 30 One object of my invention is to provide an cordance with the amplitude of the audio freimproved apparatus of and method for volume quency output. .Since the resistor I8 does not control. aect the audio frequency currents passing Another object of my invention is to provide through the windings I5 and IB, it maybe made an improved device for limiting the volume of of any desired value without affecting the said 35 output in a sound reproducer. audio frequency current. Y p y l Another object of my invention is to provide As shown in Figure 2, the sensitivity of the gas an improved volume compressor. photocell increases very greatly with an increase Another object of my invention is to provide in applied voltage. Since the resistor I8 is in 40 an improved volume compressor adapted to opseries with the source of direct current supply to f erate directly in the photocell circuit. the photocell, when the photocell current in- 40 Another object of my invention is to provide creases, due to increased modulation, the voltage a volume compressor which requires no auxiliary drop across the resistor i8 increases correspondtubes in the amplifier. ingly and the applied voltage in the photocell l0 Other and incidental objects of my invention is at the same time decreased, since only a fixed 45 will be apparent to those skilled in the art from voltage is available from the source I4. This a reading of the following specicaticn and an causes the sensitivity of the photocell III to fall inspection of the accompanying drawingin which off as the amplitude of the applied signal in- Figure 1 shows my invention as applied to a creases and the amount of this decrease in phopush-pull photocell circuit, tocell sensitivity is shown in Fig. 3, where the 50 Figure 2 is a graph of current sensitivity volume compression in decibels is plotted against plotted against anode volts on two types of phothe value ofthe resistance I8 in megohms. It tccells, will be apparent from Figure 3 that a resistance Figure 3 is a vcurve showing resistance in of 65 megohms, for example, will give a volume megohms of the resistor applied in accordance compression of 10 db. 55'

The operation of this circuit is dependent upon the fact that, in class A sound records, ground noise reduction is customarily used and the total volume of light transmitted by the track varies directly with the amplitude of the signal. If the amount of light transmitted by the track is controlled in the manner described and claimed in my application Serial No. 168,173, filed October 9, 1937, (RCV D-5374), the noise reduction envelope may be made to anticipate the signal and this will cause the volume compression to anticipate the signal, decreasing the sensitivity of the photocell just an instant before the signal arrives.

If it is desired to apply-this system of volume compression to a single photocell, this may be accomplished as shown in Figure 4. In this circuit, the photocell 20 is` connected to the source of current M in kthe same manner as described for `Figure 1 and the output of the circuit is connected to an appropriate transformer 2l.v This circuit diiers, however, from that of Figure 1 in that there would be a tendency for the direct current to pass through the primary of the transformer 2l and a blocking condenser 22 is accordingly inserted in series with this primary, while the primary is shunted by means of the resistor 23 which serves to carry the direct current. The resistor i8 is also shunted by a byv:"j pass condenser 2li so that the said resistor need not carry any audio frequency current. 'Ihe circuit otherwise operates in the same manner as the circuit shown in Figure 1 and described above.

In this latter system, the volume compression,

' like that in the system shown in Figure 1, de-

Y Having now described my invention, I claim:

1. Apparatus of the class described including a push-pull photocell, a push-pull output circuit, power supply means connected from an electrode of said cell to the midpoint of said output circuit, and a resistance of not less than 20 megohms between said power supply means and said midpoint.

2. Apparatus of the class described including a push-pull photocell, a push-pull output circuit, power supply means connected from one electrode side of said cell to the midpoint of said output circuit, and a resistance of not less than 20 megohms between said power supply means and said midpoint.

3. Photoelectric apparatus including a` light translating meansv having an anode and a cathode, power vsupply means connected to the anode of said light translating means, an output coupling device connected to the cathode of said means, and resistance of `not less than 20 megohms between said power supply and said output coupling device.

4. Photoelectric apparatus including a light translating means having an anode and a cathode, power supply means connected to one electrode side of said light translating means, an output coupling device connected to the other electrode side of said means, and resistance of not less than 2O megohms between said power supply and said output coupling device for producing volume compression.

5. In combination, a photocell, an output transformer, a power supply connected to the anode of said photocell, the cathode of said photocell being connected to said transformer and a resistance of not less than 20 megohms connecting the negative terminal of said power supply and said output transformer,

6. In combination, a photocell, an output transformer, a power supply connected to one electrode side of said photocell, the other electrode side of said photocell being connected to said transformer and a resistance of not less than 20 megohms connecting the negative terminal of said power supply and said output transformer.

GLENN L. DIMIMICK. 

